Re: Feature proposal for JDO 2.1 maintenance: current DB time

Wes Biggs Tue, 24 Oct 2006 06:31:39 -0700

For what it's worth, JPA defines

CURRENT_DATE
CURRENT_TIME
CURRENT_TIMESTAMP


all of which return datetime.

I don't really like these as they're very relational. But I think theremay be confusion (as Matthew notes) if the function is getDate() -- doesit return a date, day of month, timestamp (and if so, with whichportions zeroed), or what?

I'm also not fond of "new Date()" or "System.currentTimeMillis()",because I don't know if it will be clear to the user *when* thesestatements will be executed.


I could probably live with something like "currentTimestamp()".

Wes

Jörg von Frantzius wrote:

Just to add one more to the list of undecidable possibilities: howabout "System.currentTimeMillis()"? That would be similar to what youcan use in Java, and it is fairly self-explanatory.

Not sure whether the result being a long makes this a handy candidate,but maybe there is some easy way to convert this to a Timestamp orDate object that I don't know off the top of my head...


Regards,
Jörg

Erik Bengtson schrieb:

Good to know.  The point that I made on the conf call was that
"JDOHelper.getDate()" in JDOQL would be inconsistent with the actual API.
Java's Date.getDate() returns an int in the range 1-31.

It's for that reason that I'm advocating a JDOQL function instead.  I propose
date() & DATE() for this.


Makes sense. Another contrainst to consider is that some DBs will have dates
including timezones and others dont, the behavior on comparisons should be
unspecified and may differ from db to db.

Quoting Matthew Adams <[EMAIL PROTECTED]>:

--matthew

-----Original Message-----
From: Erik Bengtson [mailto:[EMAIL PROTECTED]
Sent: Friday, October 20, 2006 12:07 PM
To: jdo-dev@db.apache.org; [EMAIL PROTECTED]
Subject: RE: Feature proposal for JDO 2.1 maintenance: current DB time

I don't think that aligning with JPOX's JDOQL extensions is

a goal of the

spec. :)

We can always try :)

I'm happy with any expression, date(), currentDate(), Date.getDate(),
JDOHelper.getDate(), etc.


Quoting Matthew Adams <[EMAIL PROTECTED]>:

Hi Erik,

I don't think that aligning with JPOX's JDOQL extensions is

a goal of the

spec. :)

While Date has methods like getDay(), getMonth(), etc., and

Date's getDate()

method returns the day of the month between 1 & 31 (and is

deprecated in

favor of Calendar.get(Calendar.DAY_OF_MONTH)).

The only native syntax in Java to do this is "new Date()",

but we know that

might have problems with JDOQL's constructor syntax.  This is why I
recommended the standalone function currentDate(), similar

to avg, count,

etc.

To align with upper and lower casing conventions, I guess I

would recommend

"date()" and "DATE()" to avoid the camel casing.

--matthew

-----Original Message-----
From: Erik Bengtson [mailto:[EMAIL PROTECTED]
Sent: Friday, October 20, 2006 11:42 AM
To: jdo-dev@db.apache.org
Cc: [EMAIL PROTECTED]
Subject: Re: Feature proposal for JDO 2.1 maintenance:

current DB time

Hi,

I like the currentDate() and dislike adding methods to the PM.
Methods unrelated
to management of PCs lying in PM does not sound good to me,
but currentDate in
particular fits very well in JDOQL.

JPOX has several additional JDOQL functions. Similar to
currentDate(), we have
Date.getDay(), Date.getMonth(), etc...
http://www.jpox.org/docs/1_1/query_jdoql_methods.html

I would propose Date.getDate() which aligns to JPOX JDOQL

methods. :)

Regards,

Quoting Michael Bouschen <[EMAIL PROTECTED]>:

Hi,

another alternative is making it a keyword, so something like
CURRENTDATE or CURRENT_DATE w/o parenthesis.

I agree with leaving the resolution of the return value

unresolved.

Regards Michael

As we discussed on the Fri conf call today, I think that a

cleaner solution

is to introduce a new function "currentDate()" to JDOQL for

this, similar to

JDOQL's count/sum/min/max/avg.  Further, I would prefer to

leave unspecified

the resolution of the return value for this (second,

tenth of second,

millisecond, etc).

As for the Bin Sun's proposal to add a method to the API

for this purpose,

I think that it's interesting.  I would prefer that it be

placed on

PersistenceManager with the signature "Date

getCurrentDate()".  It could be

specified that if the underlying datastore cannot support

this functionality,

the implementation will return null.  I considered

specifying that the

implementation should return "new Date()" in the event the

datastore didn't

support it, but I think returning null is more informative

to the caller.  It

clearly indicates that the caller just can't know the

answer to that

question, and the JVM's current datetime may be different

enough from the

underlying database's datetime that using that value may be

unreliable.  The

question of the datetime's resolution remains for this method.

--matthew

-----Original Message-----
From: Bin Sun [mailto:[EMAIL PROTECTED]
Sent: Friday, October 20, 2006 1:59 AM
To: Michael Bouschen; jdo-dev@db.apache.org
Cc: Jörg" von Frantzius; JDO Expert Group
Subject: Re: Feature proposal for JDO 2.1 maintenance:

current DB time

Hi, all!

   I think to least impact the current API, we'd
better simply add a method to PMF or PM:
   public Date getNow();

   Then we can use this Date as a parameter in any
Query.

--- Michael Bouschen <[EMAIL PROTECTED]> wrote:

Hi,

I see one issue with using the syntax "new Date()":
it might conflict
with the constructor expression used in the result
expression, because
both use the keyword "new". A constructor expression
is only supported
in the query result expression; it is used to wrap
values into a result
class element and it is evaluated on the client. The
expression "new
Date()" should be supported in the result and the
filter, it does not
wrap any other values and it is evaluated on the
database back end.
How about using a different syntax for the current
database date, e.g.
JDOHelper.CURRENT_DATE() ?

The other interesting issue with the example from
Jörg is that it does
not define a candidate class. I think this was on
purpose, because the
query should not access any tables in the database,
but just return the
current date. Maybe we could use the JDOHelper as
the candidate class in
this case? This would be something similar to the
DUAL table in oracle.

Regards Michael

Your 1st requirement is to retrieve the date on

the client. A 2nd requirement

would be comparisons over database server date.

I have one sample for the 2nd requirement

Query query = newQuery("select from Person where

startDate > new Date()");

Quoting Jörg von Frantzius

<[EMAIL PROTECTED]>:

Hi Craig,

I'm not so sure whether this is really what you

want to see, but here's

something:

    Query query = newQuery("new Date()");

query.setResultClass(java.sql.Timestamp.class);

    query.setUnique(true);
    Date result = (Date)timeQuery.execute();


That result Date can then be used to e.g. set an

updated object's

lastModification timestamp before committing it.

Regards,
Jörg

Craig L Russell schrieb:

Hi Jörg,

Sorry to exercise you more on this, but I'm

still having a bit of

difficulty seeing how to use this feature.

Could you please give us an example of the use

case you describe

below? I'd like to see the JDOQL query that uses

new Date() in action.

Thanks!

Craig

On Oct 19, 2006, at 1:33 AM, Jörg von Frantzius

wrote:

Hello Craig,

as far as I can see that does satisfy our

requirements. Once we are

able to query for that date in JDOQL, we can

use it e.g. for

lastmodification timestamps and the like.

Regards,
Jörg

Craig L Russell schrieb:

It's easy enough to define "new Date()" as

being evaluated on the

back end for queries that are executed on the

back end. And being

evaluated in the vm for queries that have a

bound candidateCollection.

But does this satisfy the requirements? Once

you have a Date in

JDOQL, what can you do with it?

Craig

On Oct 16, 2006, at 11:58 AM, Erik Bengtson

wrote:

+1. maybe "new Date()" could be the

expression where evaluation

occurs on the
database.

Quoting Jörg von Frantzius

<[EMAIL PROTECTED]>:

Dear experts,

there had been several occasions where in

our applications we had to

determine the database server's current

time(-stamp). In one

application
we needed it to synchronize sent JMS

messages with visibility of

commits
in the database, and in another we need it

for our simple replication

algorithm.

In distributed systems in general it is

often crucial for

synchronization purposes to have a common

source of time information

that is accessible from all processes.

It would be great if JDO2 could offer a way

of doing that

independently
of the database, e.g. as a JDOQL function.


Regards,
Jörg

Craig Russell
Architect, Sun Java Enterprise System

http://java.sun.com/products/jdo

408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!

Craig Russell
Architect, Sun Java Enterprise System

http://java.sun.com/products/jdo

408 276-5638 mailto:[EMAIL PROTECTED]
P.S. A good JDO? O, Gasp!

--
Michael Bouschen                [EMAIL PROTECTED] Engineering GmbH
mailto:[EMAIL PROTECTED]        http://www.tech.spree.de/
Tel.:++49/30/235 520-33         Buelowstr. 66
Fax.:++49/30/2175 2012          D-10783 Berlin

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around
http://mail.yahoo.com

--
Michael Bouschen                [EMAIL PROTECTED] Engineering GmbH
mailto:[EMAIL PROTECTED]        http://www.tech.spree.de/
Tel.:++49/30/235 520-33         Buelowstr. 66
Fax.:++49/30/2175 2012          D-10783 Berlin

Re: Feature proposal for JDO 2.1 maintenance: current DB time

Reply via email to